AN2385 datasheet(5/14 Pages) STMICROELECTRONICS

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AN2385

Determination of the SOA limits

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Determination of the SOA limits

SOA is the acronym of Safe Operating Area. It includes all the ID-VDS operating points

inside where the device works in safety conditions. There are two kinds of SOA. The first

one is named Forward Biased Safe Operating Area (FBSOA), while the second one is

named Reverse Biased Safe Operating Area (RBSOA). FBSOA is the SOA during the

device on state, while RBSOA is the SOA when the MOSFET switches off. Supposing that

the ID and VDS axis are in log scale, a typical FBSOA can be depicted as in Figure 1.

Figure 1.

Typical FBSOA of a Power MOSFET

IDmax is the maximum drain current limit of the MOSFET. It is usually fixed by the wires that

connect the drain and source pads to the package pins respectively. BVdss is the maximum

drain-source voltage that the device can sustain (breakdown voltage). Pmax(t) is the

maximum power that the device can dissipate. It depends on the junction temperature and

power pulse interval time and on the package used. In fact, if the junction temperature

overcomes typically 150'C or 175'C, as defined in the automotive devices, the MOSFET

could fail or, however, the device works out of the guaranteed temperature spec.

Furthermore, increasing the case temperature, the Pmax(t) value decreases due to a lower

energy, which is necessary to bring the junction temperature to the maximum guaranteed

value. Pmax is also a function of the power pulse interval time because, fixing the power

pulse, the energy dissipated in the MOSFET, as well as the junction temperature rises,

increasing the power interval time. In particular, when the power pulse interval time

increases, Pmax(t) decreases and the SOA area decreases too.

Another limit for FBSOA is established by RDSON of the device. In fact, when the device is in

on state without loads connected to the component, all the voltage feed is applied on the

drain-source terminals and, thus, the maximum drain current that can flow in the transistor is

fixed by VDD and RDSON (Figure 2.).

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